The invention relates to a linearization device for an amplifier with a broad frequency band, notably in the UHF range.
When the power requirement of electronic signals are high, recourse is had to an amplifier which is made to operate close to its saturation power. By way of example, the transmission requirements of telecommunications satellites, for which amplifiers with a progressive-wave tube or tubes are often used, can be cited here.
Operating close to saturation causes a non-linear response of the amplifier. More precisely, when the input power is substantially lower than the saturation power, the output power is substantially proportional to the input power. On the other hand, when the input power is close to the saturation power, the gain decreases, and takes the value 1 at saturation. This lack of linearity also has an effect on the phase of the output signal: this phase is constant when the amplifier is operating outside the saturation region and, for certain amplifiers (notably those with a progressive-wave tube), decreases when the power is close to the saturation value.
To remedy this drawback, use is normally made, notably for tube amplifiers for which the gain and phase decrease when the input power approaches the saturation value, linearization devices with predistortion. Such a device is disposed upstream from the amplifier or tube to be linearized. It supplies an output signal whose amplitude and phase vary according to the input power in a non-linear manner so that the amplitude and phase of the output signal of the amplifier vary non-linearly as a function of the power of the signal at the input of the predistortion device.
More precisely, in the linearization device, there is applied, both to the gain and to the phase, a predistortion which keeps the gain and phase constant for values of the input power which are substantially less than the saturation power and a gain and phase which increase when the input power approaches the saturation power, which makes it possible to compensate for the reduction in gain and phase of the amplifier.
The predistortion devices known up to the present time which can be used for space applications operate correctly only for frequency bands with relatively low values, for example from 250 to 500 MHz, in a frequency range of between 10.7 and 12.75 GHz.
Linearization devices for amplifying tubes are also known which can operate over a wide frequency band, notably a band from 1.5 to 2 GHz in Ku band. However, the known devices have complex structures using several UHF integrated circuits. They are tricky to adjust and are sensitive to temperature. In addition, they generally have a high price and are bulky, which constitutes a serious drawback for many applications, notably space applications.
The device according to the invention allows linearization over a wide frequency range. It can be produced at low cost and by means of a UHF integrated circuit technology.
The linearization device according to the invention, which operates in the UHF range, comprises an input coupler separating this input signal into two in-phase signals, one of which is delivered to a first channel with a Schottky diode connected in series and the other to a second channel with at least one Schottky diode connected in parallel, the output signals of the two channels being rephased by an output coupler, preferably a 180xc2x0 coupler.
The channel with a Schottky diode in series confers phase predistortion. On the other hand, it reinforces the distortion of the gain. The channel with a Schottky diode or diodes in parallel keeps the phase constant, but increases the gain. The latter compensates for the parasitic distortion of gain introduced into the channel with a diode in series and provides, at least partially, the necessary predistortion on the gain.
It is preferable for the channel with a diode connected in parallel to have at least two Schottky diodes connected in parallel.
In the preferred embodiment, the output of the coupler, notably at 180xc2x0, is connected to the output of the predistortion device by a third channel with at least one Schottky diode connected in parallel.
This third channel supplements the predistortion of the gain obtained by the second channel, also with Schottky diode or diodes connected in parallel. It was found that the best results were obtained by providing two diodes connected in parallel in this third channel.
It is preferable to apply continuous biasing voltages which can be adjusted on the diodes in each channel, so as to permit, in the channel with a diode connected in series, an adjustment of the phase predistortion and, in the channel or channels with diode or diodes connected in parallel, an adjustment of gain predistortion.
When an output coupler is provided for rephasing the signals supplied by the first and second channels, it is advantageous to associate it with an adjustment element, such as a xe2x80x9cstubxe2x80x9d, which maintains the rephasing of the signals of the first two channels in spite of variations in frequency and power.
In the second channel having at least one Schottky diode connected in parallel, it is also possible to provide another Schottky diode connected in series, upstream from the diode (or diodes) connected in parallel, this diode in series being able to improve the gain predistortion.
Thus the invention provides an amplifier linearization device of the predistortion type which comprises two channels each receiving the input signal, the first conferring a phase predistortion and the second conferring a gain predistortion, the two channels supplying a coupler for phasing the output signals on these two channels.
In the preferred embodiment, the rephasing coupler is connected to the input of the amplifier to be linearized by means of a supplementary channel permitting a complementary adjustment of the gain and/or phase predistortion.
The present invention provides a device for linearizing the gain and phase of an amplifier operating in the UHF range comprising means for conferring a predistortion of the gain and phase which compensates for the non-linearities in gain and phase of the corresponding amplifier, notably of the type with electronic tube or tubes, and which has a first channel receiving the input signal and has a Schottky diode connected in series, a second channel which also receives the input signal and has at least one Schottky diode connected in parallel, and a coupler for phasing the signals supplied by the first and second channels.
According to one embodiment, the second channel has two Schottky diodes connected in parallel.
According to one embodiment, the device has a third channel, connected at the output of the coupler and comprising at least one Schottky diode connected in parallel.
The third channel can also have two Schottky diodes connected in parallel.
According to one embodiment, each of the channels has an input for continuous biasing for adjusting the gain and phase predistortions.
This continuous biasing is for example applied by means of an adjustable adjustment resistor.
According to one embodiment, the coupler intended to rephase the signals delivered by the first and second channels is of the 180xc2x0 type, for example with three transmission lines.
According to one embodiment, the device has an input coupler with two outputs in phase to which there are connected respectively the first and second channels.
This input coupler is for example of the Wilkinson type.
According to one embodiment, the device has a member for keeping the phase of the output coupler constant in spite of the variations in input power and frequency.
According to one embodiment, the Schottky diode is connected in series in the second channel upstream from the diode or diodes connected in parallel.
According to one embodiment, all the diodes are identical.
The invention also provides for an application of the device to the linearization of an electronic-tube amplifier intended to be mounted aboard a space vehicle, notably a telecommunications satellite.